The images of the Deepwater Horizon drilling rig engulfed in flames and sinking into the Gulf of Mexico emphasised the importance of Situation Awareness (SA) for safety in offshore drilling in a way that human factors research had not previously managed. The purpose of the thesis was to identify the cognitive components associated with offshore drillers' SA, to apply Endsley's three level model of SA (1995) to the drilling domain, and to design interventions that support drillers' awareness. Interviews with experienced drill crew (n=18), live observations and video analysis (n=24hrs) of drilling simulator scenarios identified key drillers' SA components and factors that can positively and negatively influence driller's SA. These findings were used to develop the preliminary Drillers' Situation Awareness (DrSA) model. A cognitive task analysis classified the technical steps (well control manuals) and SA components (using the DrSA model) required for the well control task of kick detection. An analysis of a critical phase of the Deepwater Horizon drilling disaster was conducted by applying the DSA model to the cognitive data extracted from investigation reports and court hearing transcripts, revealing issues with the drill crew's awareness and possible influencing factors. Finally, two interventions to support drillers' SA were designed: the computer-based Drillers' Situation Awareness Task (DSAT) developed to measure aspects of SA and Decision Making (DM). It was found to be potentially viable as a measurement tool from a pilot study (n=14). A prototype behavioural marker system for drillers' SA and DM (DCogs) was developed from prior cognitive task analysis data, refined and behavioural markers identified using a review panel. The system was found to preliminarily have an acceptable content validity using an online survey (n=15) of drilling personnel. The performance measures of the DSAT and DCogs have direct applications for supporting drillers' SA and DM through training and assessment.